1. Field of the Invention
The present invention relates generally to Radio Frequency systems, and more particularly to low-noise control of Radio Frequency devices without the use of free-running clocks.
2. Related Art
In modern integrated avionics systems, digital devices such as signal processors, data processors, and controllers, are often used to control Radio Frequency (RF) devices such as receivers, transmitters, and antenna electronics. These systems are deployed in a wide variety of avionics applications, including both ground and airborne environments for military and commercial users. Digitally controlled RF devices also find use outside of avionics, such as in the telecommunications industry. For example, cellular telephone base stations often include RF devices controlled by digital hardware.
The task of controlling RF devices using digital hardware is difficult. The digital hardware must provide the necessary control without coupling digital noise into the sensitive RF circuitry. Furthermore, precise timing of execution of commands by the RF devices is critical. Yet, in order to minimize digital noise, many RF devices operate without a free-running clock. As a result, these devices have no sense of time.
A critical component of any RF control system is the bus over which commands and data are sent. Most commercially available data buses emphasize high speed and throughput or long distance runs, neither of which are required for RF control. Data busses also tend to generate more digital noise, which makes them undesirable for RF control use.
Conventional serial communications bus techniques are based on a clock pulse data scheme. These techniques may provide clock and data signals on separate lines or provide a data signal which is encoded with clock information. Alternatively, data may be transmitted asynchronously with a clock signal being generated locally by the receiver. All these techniques require that the receiver view data at the specific instant in time associated with a clock edge. None of these techniques provides RF control with sufficiently low noise to operate with highly sensitive RF devices.
A need therefore exists for an improved system and method for low-noise digital control of RF devices without the use of free-running clocks.
Briefly stated, the present invention is directed to a system and method for controlling RF devices, such as receivers, transmitters, and antenna electronics. According to the present invention, a serial RF control bus provides a half-duplex serial communication interconnect path between a bus master and one or more bus slaves. The bus master is coupled to a processor, and each bus slave is coupled to an RF device that operates without a free-running clock. The processor controls the RF devices by sending and receiving messages over the RF control bus. The bus master and bus slaves format these messages for transmission across the RF control bus. The control bus includes a data line, a first clock line, and a second clock line. The first clock line is asserted by the bus master when transmitting serial data to and receiving serial data from the RF slaves via the data line. The second clock line is asserted by the RF slaves when transmitting serial data to the bus master via the data line.
An advantage of the current invention is that low-noise control of RF devices is achieved with a minimum number of differential interconnects and without using free-running clocks, thereby minimizing a significant source of electromagnetic coupling. RF devices of increased sensitivity can be controlled as a result.
Another advantage of the current invention is that all knowledge of time is in the hands of the bus master. This provides centralized control of time of execution of all commands, and frees the bus slaves and/or RF devices from any time keeping requirements.